Poly(amic-acid)s are typical polyimide precursors and have been produced, for example, by a low temperature solution polymerization method in an aprotic polar solvent such as N,N-dimethylformamide, N-methyl-2-pyrrolidone, hexamethylphosphoramide, N-methylcaprolactam, dimethyl sulfoxide, N-acetyl-2-pyrrolidone, N,N-dimethylacetamide or the like. In this instance, each of these aprotic polar solvents strongly associates with a polyimide precursor which is the solute in the resulting solution because of the high dipole moment of the solvent. Such a phenomenon has been indicated for instance in J. Polymer Sci. (A-1, vol.4, pp.2607-2616, 1966), J. Polymer Sci. (A, vol.25, pp.2005-2020, 1987), J. Polymer Sci. (A, vol.25, pp.2479-2491, 1987) and ANTEC 91 Abstract (pp.1742-1745).
As described above, because of the strong solvent-solute mutual interaction, there is presented a problem in that the solvent cannot be easily removed when a polyimide precursor, a polyimide molding, a polyimide coating or the like is prepared from a polyimide precursor solution which has been obtained by a low temperature solution polymerization method. In addition to this, there is another problem in that hydrolysis of a polyimide precursor occurs due to the hygroscopic nature of the aprotic polar solvent, resulting in a reduction of the degree of polymerization of the polyimide precursor (J. Appl. Polymer Sci., vol.8, pp.1039-1051, 1964).
With the object of overcoming such problems, processes have been developed as disclosed in Kogyo Kagaku Zasshi (vol.71, No.9, pp.1559-1564, 1968) and J. Appl. Polymer Sci. (vol.11, pp.609-627, 1967), in which a polyimide precursor is formed by a polymerization reaction using a solvent, such as tetrahydrofuran, which has a low boiling point and can be easily removed. According to these processes, a polyimide precursor which does not contain an aprotic polar solvent can be obtained, but the intrinsic viscosity of the product is merely 0.5 to 0.6, and a polyimide molding prepared from the polyimide precursor does not show sufficient mechanical properties.
Other type of processes have been disclosed in JP-B-39-22196, JP-B-39-30060, JP-A-60-221425, JP-A-61-234, JP-A-61-250030 and JP-A-2-18420, in which a polyimide granular material is produced by adding a poor solvent, such as acetone, ethyl acetate or the like, and a cyclization agent, such as pyridine or the like, to an aprotic polar solvent of a polyimide precursor. (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively) However, since most of the repeating units in these polyimide granular materials are imidated, direct molding of these granular materials results in considerably poor thermal fluidity, and the resulting polyimide moldings do not show sufficient mechanical properties. In addition, such granular materials cannot be made into solution form because of their considerably low solubility in organic solvent.